INTRODUCTION

Desmoscolecida Filipjev, 1929 is a small group of free-living nematodes with a widespread distribution, and generally display high species diversity but low abundance. To date, about 270 species of 24 genera in three families have been reported worldwide (Freudenhammer 1975;Decraemer and Smol 2006;Decraemer and Rho 2013;Bezerra et al. 2021). Most species have been described from marine environment, and only 24 species from salt marshes, freshwater and terrestrial habitats. In general, desmoscolecid nematodes are easily recognized because majority show transverse stripes or strong body rings covered with desmens on the epidermis of the body (Decraemer and Smol 2006;Decraemer and Rho 2013).

The genus Quadricoma was first established by Filipjev in 1922 based on the type species of Q. loricata discovered in Great Barrier Reef, Australia. The genus QuadricomaFilipjev, 1922 was classified as a subgenus of the genus Tricoma by Decraemer (1985), which belongs to subfamily Tricominae Lorenzen, 1969. All species of the taxa are distributed worldwide, and discovered in various marine environments, ranging from intertidal to deep-sea habitats (Decraemer and Rho 2013). Quadricoma can be distinguished from other genera of the family in having diagnostic features such as desmens with a triangular outline, an abrupt reversal of direction in the inversion ring, end ring often with fine offset spinneret, and more rounded head shape with flatted anterior end (Decraemer 1985, 1998;Decraemer and Rho 2013;Bezerra et al. 2021).

In Korean waters, only one Quadricoma species of T. (Q.)jindoensisLim and Chang, 2005 has been recorded from sublittoral bottom sands and the various particulate substrates on algal mats at rocky shore of Jindo Island to date. During a continuous faunistic survey on the biodiversity of the free-living marine nematodes around the Korean waters, two new Quadricoma nematodes were collected from the washings of a subtidal sediment in the East Sea, and intertidal sediments in Jeju Island. We provide a taxonomic description of the two new species with illustrations and photomicrographs using a scanning electron microscope (SEM) and a differential interference contrast (DIC) microscope, in addition to illustrated pictorial key and character comparisons tables of all valid species within the subgenus Quadricoma.

MATERIALS AND METHODS

Sampling of nematodes

Sediment samples were collected from upper surface of the intertidal sandy beach in Jeju Island of Korea using a hand scoop, and also taken by means of the Smith-McIntyre Grab from subtidal benthic sediment at 125 m depth of the East Sea.

Sample processing and preparation of specimens

Sediment samples were initially fixed in formaldehydeseawater solution at a final concentration of 5% for longterm preservation. In the laboratory, a Ludox HS40 was used to extract meiobenthos from detritus and sediments screened through a 63 μm mesh-sieve with three centrifugation cycles (Burgess 2001). Nematodes were sorted from mixed meiobenthos under high magnification of a dissecting microscope (LEICA 205 C; Leica, Germany). Individual nematodes to be targeted were picked out and transferred to a 5% glycerin solution. After slowly evaporating the water for ten days at room temperature, they were mounted between two cover slips on a HS slide using the standard wax-ring method (Shirayama and Higgins 1993).

Light microscopy and scanning electron microscopy

Mounted specimens were measured, examined and drawn using Nomarski differential interference contrast (DIC) with an Olympus BX53 microscope equipped with a drawing tube and an Olympus DP26 digital camera with Olympus CellSens corresponding imaging software (Olympus, Japan). All drawings were created with tracing technique using Adobe Illustrator 2022 software. Photomicrographs were taken with a Olympus DP 26 digital camera, and image quality was improved with Adobe Photoshop 2022 software. For scanning electron microscopy (SEM), the specimen was fixed in 5% buffered formalin and rinsed twice with distilled water to remove the buffered formalin. After rinsing for five minutes, the specimens were lyophilized in a cooling stage. The dried specimens were mounted on aluminum stub sputter-coated with gold/palladium in a high-vacuum evaporator and examined with a SEC SNE-3200M Desktop Mini SEM (SEC Co. Ltd., Korea).

SYSTEMATIC ACCOUNTS

Order Desmoscolecida Filipjev, 1929

Family Desmoscolecidae Schepotieff, 1907

Genus Tricoma Cobb, 1893

Subgenus QuadricomaFilipjev, 1922

Tricoma (Quadricoma) jejuensis sp. nov. (Figs. 1-6; Table 1)

Type material. Holotype male (MABIK NA00157822), mounted in glycerin on a HS slide, are deposited in the nematode collection of the Marine Biodiversity Institute of Korea (MABIK), Seocheon, Korea. Two paratype males (KIOST NEM-1-2663, KIOST NEM-1-2664) and two paratype females (KIOST NEM-1-2665, KIOST NEM- 1-2666) were deposited in the nematode collection at the specimen conservation room of the Bio-Resources Bank of Marine Nematodes (BRBNM), East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

Type locality and habitat. Intertidal zone of Jeju Island (33°20ʹ41.82ʺN, 126°10ʹ11.28ʺE), Hangyeong-myeon, Jeju-si, Jeju-do, Korea, collected on 16 May 2019 by H. S. Rho and H. Lee. Nematodes were obtained from intertidal sandy sediments.

Etymology. This proposed specific name ‘jejuensis’ is named after the type locality of the new species.

Measurements. See Table 1 for measurements and morphometrics.

Diagnosis. Body elongated with 33 quadricomoid main rings; inversion at main ring 23. Pentagonal head with truncated anterior end; cephalic setae relatively shorter than head diameter and inserted on low peduncles. A pair of ocelli situated at main ring 6. Somatic setae comprising of 8 pairs of subdorsal setae and 12 pairs of subventral setae. Males with relatively short spicules of 42-46 μm. Tail composed of five main rings; terminal ring with spinneret (10-11 μm long) and round phasmata.

Description. Males (holotype and paratypes). Body length 590-618 μm long, slender and elongated, ventrally curved, slightly tapered towards both extremities. Maximum body diameter at mid body level 53-59 μm wide. Cuticle composed of 33 typical quadricomoid main rings with inversion at level of main ring 23 (Figs. 1A, 3A, 5A). Each main ring consisting of secretion and fine foreign particles (Figs. 1B, 3C). Head pentagonal, 21-24 μm wide, 1.2-1.4 times wider than long, anteriorly tapered from level of cephalic setae with truncated end, and posteriorly with more or less cylindrical neck-region (Figs. 1B, 3B, 5B). Head cuticle completely covered with thin layer of fine granular, except in amphideal zone (Fig. 5C). Anterior margin of head cuticle strongly thickened and sclerotized, distinctly strengthening border. Labial region with 6 slightly protruding lips, composed of weak cuticle. Cephalic setae tapering distally, 20-21 μm long, slightly shorter than head width, inserted on broad and low peduncle in middle of head. Amphideal fovea rounded, covering head laterally (Figs. 1C, 5C). Stoma small and cylindrical. Oesophagus cylindrical, 70-82 μm long, extending to level of posterior margin of desmens 4-5. A pair of ocelli dark-yellowish, relatively small oval shape (5-7 μm wide, 5-9 μm long), and situated between main ring 6 (Fig. 1A). Somatic setae relatively long with vary fine mid-central canal, gradually tapered to corresponding tip, and inserted directly without peduncle on body cuticle. Somatic setae composed of 8 pairs of subdorsal setae and 12 pairs of subventral setae, with some setae cut or destroyed, and setae arrangement somewhat variable depending on individuals. Somatic setal arrangement as follows (number in parentheses means paratypes variable location): subdorsal, left side: 4, 7, (10)11, (13)14, 17, 21(22), 26, 31=8, right side: 4, 7, 11, 14, 17, 21, 25(26), 31=8; subventral, left side: 3, 6, 8, 9(10), 12, 14, 16(17), 19, 22, 25, 28, 32=12, right side: 3, (5)6, (7)8, 10, 12, 14, 16, 19, 22, 25, 28, 32=12. Reproductive system typical of subgenus, anterior testis reaching to main ring 20. Spicules short and equal, slightly curved, with distally tapered to a pointed tip, and proximally with a weakly developed offset capitulum. Spicules 42-46 μm long, and about 0.9-1.0 times of cloacal body diameter (Figs. 1D, 5D). Tail consisted of five main rings (Fig. 3D). Terminal ring 42-49 μm long, 41-45% of total tail length, consisting of broad conical anterior part covered with foreign material, and elongated spinneret (10-11 μm long).

Females. Similar to males in most respects but differ in relatively longer body length and sexual characteristics (Figs. 2B, C, 4B, 6B, C). Body length 674-690 μm long, and maximum body diameter 60-73 μm (Figs. 2A, 4A, 6A). Body cuticle with 33 main rings, inversion at level of main ring 23 (Fig. 4C). Somatic setae with 7 pairs of subdorsal setae and 10 pairs of subventral setae, arranged as follows (numbers in parentheses means paratypes variable location): subdorsal, left side: 4, 7(8), 10(11), 13(14), 17, 22, 26, 31 (32)=8, right side: 4, 7(8), 11, 14, 17(18), 21(22), 26(27), 31=(7)8; subventral, left side: 3, 6, 8, 10, 12, 14(15), 17, 19, 22, 25, 28 (29), 32=(11)12, right side: 3, 5(6), 7(8) 10, (12), 14, 16, 19, 22, 25, 28(29), 32=11(12). Reproductive system didelphic-amphidelphic with both branches outstretched. Vulva situated between main rings 19 to 20, or 20 to 21. Anal tube protruded from medioventral main ring 29. Tail composed of five main rings. Terminal ring 60-66 μm long, about 2.5-2.7 times longer than wide, consisting of broad conical anterior part covered with foreign materials, and uncovered spinneret (11 μm long) (Figs. 2D, 4D, 6D).

For differential diagnosis and relationships, see Discussion.

Tricoma (Quadricoma) unipapillata sp. nov. (Figs. 7-12; Table 2)

Type material. Holotype male (MABIK NA00157823), in glycerin on a HS slide, was deposited in the nematode collection of the Marine Biodiversity Institute of Korea (MABIK), Seocheon, Korea. Two paratype males (KIOST NEM-1-2668, KIOST NEM-1-2669) and two paratype females (KIOST NEM-1-2670, KIOST NEM-1-2671) mounted on HS slides were deposited in the nematode collection at the specimen conservation room of the Bio-Resources Bank of Marine Nematodes (BRBNM), East Sea Research Institute, Korea Institute of Ocean Science Ȇ Technology (KIOST), Korea.

Type locality and habitat. Subtidal zone of the East Sea (37°52ʹ45.93ʺN, 128°55ʹ53.37ʺE), Sacheon-myeon, Gangneung- si, Gangwon-do, Korea, collected on 17 April 2018 by H. G. Kim, H. J. Lee and H. Lee. Nematodes were obtained from subtidal sediments with tiny shell gravels and detritus collected at a depth of 124 m using Smith-McIntyre Grab.

Etymology. This specific name ‘unipapillata’ is derived from Latin word for a single papilla in the subventral body region of the main ring 21-23.

Measurements. See Table 2 for measurements and morphometrics.

Diagnosis. Body composed of 44 quadricomoid body rings with inversion at level of main ring 31. Globular head with broadly truncated anterior end; six lips slightly protruding anterior margin. Cephalic setae relatively short and stumpy, tapering into fine open tip and cuticular flange. Somatic setae comprising of 7 pairs of subdorsal setae and 10 pairs of subventral setae. Single stout ventral median genital papillae situated on main ring 21-23. Spicules relatively short and proximally with a marked capitulum. Tail composed of eight main rings in males and seven main rings in females; terminal ring with elongated spinneret (7-9 μm long) and round phasmata.

Description. Males (holotype and paratypes). Body length 564-596 μm long, slender and elongated, ventrally curved, slightly tapered towards both extremities. Maximum body diameter at mid-body level 63-76 μm wide. Cuticle composed of 44 typical quadricomoid main rings with inversion at level of main ring 31 (Figs. 7A, 9A, 11A). Each main ring consisting of secretion and fine foreign particles, without interzones. Head diameter 15-18 μm wide, 1.1-1.3 times wider than long, anteriorly tapered from level of cephalic setae with truncated end, and posteriorly with more or less cylindrical neck region. Anterior margin of head cuticle strongly thickened and sclerotized, forming distinctly rim-shaped border. Labial region with 6 slightly protruding lips, composed of relatively thin weak cuticle. Cephalic setae 12-13 μm long, shorter than head width, inserted on broad and low peduncle in middle of head (Fig. 7B). Cephalic setae with broad and cylindrical basal part, tapering into fine open tip, and whole setae flanked by cuticular flange (Figs. 7A, 9B, 11B). Amphideal fovea large, almost completely covering lateral side of head; anteriorly extending just before protruding lips, and posteriorly reaching upper edge of the first cuticular ring. Amphideal canal ending in small groove near posterior margin of head (Figs. 7C, 11C). Stoma small and cylindrical. Oesophagus cylindrical, 85-87 μm long, about 11-14% length of total body length, 52-56 μm corresponding body diameter. Oesophagus surrounded by nerve ring at level of main rings 4-5. Oesophagus-intestinal junction occurred at main ring 8. A pair of ocelli dark-yellowish, oval-shaped (11-22 μm long), and showing variable size and location. Somatic setae short and stumpy, distally with a groove seeming splitted tip, and inserted almost directly into cuticular rings without peduncle. Somatic setae with 7 pairs of subdorsal setae and 10 pairs of subventral setae, but setae arrangement somewhat variable depending on individuals. Somatic setae arrangement as follows (number in parentheses means paratypes variable location): subdorsal, left side: 3, (8)9, (14)15, 20, 26(27), 33(34), 38=7, right side: (2)3, (8)9, (14, 15)16, (20)21, 26, 33, (37)39=7; subventral, left side: 2, 5, 9(10), (13)14, 18, 22, 26, (30)31, 35, 40(41)=10, right side: (2)3, (5)6, (9)10, (13)14, (17)18, (21)22, (25)26, (30)31, 35, (40)41=10. Reproductive system typical for subgenus, with two testis. Testes short, anterior one reaching to main ring 18. One single stout ventral median genital papillae situated on main ring 22 in holotype and on rings 21 and 23 in paratypes (Figs. 9C, 11D). Cloacal tube broad, clearly protruded from medioventral main ring 36. Spicules short and equal, slightly arched, with distally tapered to a pointed tip, and proximally with capitulum. Spicules 43-48 μm long, and about 1.0- 1.1 times of cloacal body diameter. Gubernaculum 20- 29 μm long, parallel to spicules, with a slightly thickened distal round tip, and proximally with 11 μm long apophysis (slightly bent posteriorly in holotype) (Figs. 7D, E, 11E, F). Tail consisted of 8 main rings. Terminal ring 26-30 μm long, 25-28% of total tail length, consisting of relatively cylindrical anterior part covered with foreign materials, and elongated spinneret (8 μm long) (Fig. 9D). Phasmata small (2.5 μm in diameter) and round, situated at about middle of terminal ring.

Females. Similar to males in most respects but differ in relatively longer body length, smaller tail body rings and sexual characteristics (Figs. 8B, C, 10B, C, 12B, C). Body length 652-687 μm long, and maximum body diameter 72-86 μm long. Body cuticle with 44 main rings (Figs. 8A, 10A, 12A). Somatic setae with 7 pairs of subdorsal setae and 10 pairs of subventral setae, arranged as follows (numbers in parentheses mean paratypes variable location): subdorsal, left side: 4, 9(10), 16, 21, (26)27, 34, 38(39)=7, right side: 4, 10, 16(17), 21(22) 27, 34, 38(39)=7; subventral, left side: (2)3, 6, 10, 14(15), 18(19), 22(23), 26, 31, 36, 41=10, right side: 3, 6, 10(11), 15, 19, 23, 26(27), (31), 36, 41=10. Reproductive system didelphic-amphidelphic with both branches outstretched. Vulva situated between main rings 25 and 26. Anal tube protruded from medioventral main ring 37. Tail composed of 7 main rings. Terminal ring 28-30 μm long, consisting of cylindrical anterior part covered with foreign materials, and elongated spinneret (7-9 μm long). Phasmata small (2.5 μm in diameter) and round, situated at about middle of terminal ring (Figs. 8D, 10D, 12D).

For differential diagnosis and relationships, see Discussion.

DISCUSSION

The genus Quadricoma was first established by Filipjev in 1922 based on the type species of Q. loricataFilipjev, 1922 discovered in Great Barrier Reef, Australia. Although the original diagnosis of the genus by Filipjev (1922) is no longer valid, the genus Quadricoma can be recognized based on the unique characteristics of the type specimens of Q. loricata (Decraemer 1978, 1998). Over the years, many nematologists have revised the taxonomic position of the genus Quadricoma. Baylis and Daubney (1926), Stammer (1935), Allgén (1942), and Lorenzen (1969) classified Quadricoma as synonyms with Tricoma, but Timm (1970), Freudenhammer (1975), and Decraemer (1978) regarded it as a valid genus. However, the taxonomic status of the genus Quadricoma within the subfamily Tricominae has long been questioned. Decraemer (1985) finally classified it as a subgenus of the genus Tricoma because some characteristics of the genus Quadricoma overlapped with those of the genus Tricoma, such as head shape, end ring and spinneret. Species identification within the subgenus Quadricoma depends on the following taxonomic characteristics: (1) number of main ring and location of inversion ring; (2) head shape; (3) shape and length of cephalic setae; (4) shape, number and arrangement of subdorsal and subventral somatic setae; (5) shape and length of spicules and gubernaculum; (6) number of tail rings; (7) ratio of length to width of end ring; and (8) shape and length of spinneret.

We herein provide a pictorial identification key and a comparison table on the diagnostic morphological characteristics for the congeners of the subgenus Quadricoma (Fig. 13; Table 3). The subgenus Quadricoma can be subdivided artificially into three species groups on the basis of the number of main rings as a major diagnostic key character (Decraemer 1998). The first species group consists of 33- 34 main rings, and contains three species as T. (Q.) avicapitataTimm, 1978, T. (Q.) loricataFilipjev, 1922, and T. (Q.) trigintatresTimm, 1970 with the present new species, T. (Q.) jejuensis sp. nov. The second species group consists of 36-39 main rings, and this species group includes eight species as T. (Q.)brevichaetaFreudenhammer, 1975, T. (Q.)cobbi (Steiner, 1916), T. (Q.)gascognensis Decraemer, 1984, T. (Q.)magnaTimm, 1970, T. (Q.)maximaSchepotieff, 1970, T. (Q.) noffsingeraeDecraemer, 1977, T. (Q.) scanica (Allgén, 1935), and T. (Q.) tripapillataSoetaert and Decraemer, 1989. The third species group is composed of 42-46 main rings, and this species group contains nine species as T. (Q.) bahamaensisTimm, 1970, T. (Q.) crassicoma (Steiner, 1916), T. (Q.) crassicomoidesTimm, 1970, T. (Q.) freudenhammeriDecraemer, 1977, T. (Q.) jindoensisLim and Chang, 2005, T. (Q.) lizardiensisDecraemer, 1977, T. (Q.) magnafenestraDecraemer, 1977, T. (Q.) papillataDecraemer, 1977, and T. (Q.) ponticaFilipjev, 1922 and including the second new species, T. (Q.) unipapllatus sp. nov.

Tricoma (Q.) jejuensis sp. nov. is closely related to T. (Q.) trigintatres mainly by possessing a similar body length and head and tail shape. However, T. (Q.) jejuensis sp. nov. differs from T. (Q.) trigintatres by the number of somatic setae in males (12 pairs of subventral setae, while 13 pairs of subventral setae in T. (Q.) trigintatres), the number of tail rings in males (5 against 4 in T. (Q.) trigintatres), and relatively short spicules length (36-46 μm vs. 72-78 μm).

Tricoma (Q.) jejuensis sp. nov. is also close to T. (Q.) avicapitata, but discernible from it by longer body length (590-618 μm vs. 350 μm in males, 674-690 μm vs. 304- 363 μm in females), different head shape (without anteriorly prolonged head region), different number of somatic setae in male (12 pairs of subventral setae, while 10 pairs of subventral setae in T. (Q.) avicapitata), and small number of tail rings in males (5 against 6 in T. (Q.) avicapitata).

Tricoma (Q.) unipapillata sp. nov. belongs to species group with 42-46 main rings, and only three species are known as possessing a ventral papilla within this group. Tricoma (Q.) unipapillata sp. nov. clearly differs from T. (Q.) bahamanensis and T. (Q.) lizardiensis by position and shape of ventral papilla. Tricoma (Q.) bahamanensis and T. (Q.) lizardiensis have one post-anal papilla completely covered with foreign materials on the tail region, while T. (Q.) unipapillata sp. nov. contains one naked pre-anal papilla situated at the main ring 21-23. Moreover, T. (Q.) unipapillata sp. nov. possesses 7 pairs of subdorsal setae and 10 pairs of subventral setae, but T. (Q.) bahamanensis and T. (Q.) lizardiensis have 9 pairs of subdorsal setae and 12-14 pairs of subventral setae.

Tricoma (Q.) unipapillata sp. nov. is morphologically resemble to T. (Q.) papillata in having a naked pre-anal papilla and similar somatic setal pattern. However, T. (Q.) unipapillata sp. nov. is clearly distinguished from T. (Q.) papillata by longer body length (564-596 μm vs. 305-395 μm in males, 652-687 μm vs. 395-555 μm in females), longer spicules length (44-48 μm vs. 25-28 μm), and the number of pre-anal papilla (1 vs. 2).